Dodoo

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- refactor to click native facilities, where possible
- replace ``env_options()`` named parameters with ``context_settings``
on ``click.Command()``
- replace ``env_options`` wrapper with custom Command class
- add ``default_overrides`` command key to manage script-scoped parameter
defaults (eg. adjust default for ``log_level`` or ``rollback``)
- Rename to dodoo
- Add plugin facilities

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- add ``with_addons_path`` option to ``dodoo.env_options``
to control the presence of the ``--addons-path`` option. Defaults to False.
Enabled for the CLI.

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- add ``environment_manager`` to ``dodoo.env_options``, providing
a hook on ``odoo.api.Environment`` creation.
- add ``--addons-path`` option to the CLI.
- add ``database_must_exist`` env option to ``dodoo.env_options``
so scripts can behave how they please in case the database is absent.

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- silence deprecation warning
- adapt tests for Odoo 12

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- clarify the behaviour of ``env_option`` ``with_database`` and ``database_required``
parameters; in particular, when ``with_database`` and ``database_required``
are both set (the default), the ``--database`` option can be omitted
as long as a database is declared in the Odoo configuration file.

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- refactor the OdooEnvironment class: it is much cleaner when
it leaves the global Odoo config alone, so we completely move
responsibility to initialize the Odoo config to the CLI part.